1. Field of Invention
This invention relates to novel, heat-developable photothermographic elements and in particular, it relates to ribonucleic acids (RNA) as antifoggants and post-processing print stabilizers for photothermographic elements.
2. Background of the Art
Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic elements) processed with heat, and without liquid development, have been known in the art for many years. These elements are also known as "dry silver" compositions or emulsions and generally comprise a support having coated thereon: (a) a photosensitive compound that generates silver atoms when irradiated; (b) a non-photosensitive, reducible silver source; (c) a reducing agent (i.e., a developer) for silver ion, for example the silver ion in the non-photosensitive, reducible silver source; and (d) a binder.
The photosensitive compound is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver atoms (also known as silver specks, clusters, or nuclei) are generated by irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that silver atoms (Ag.degree.) are a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions. The silver halide may be made "in situ," for example by adding a halogen-containing source to the reducible silver source to achieve partial metathesis (see, for example, U.S. Pat. No. 3,457,075); or by coprecipitation of silver halide and the reducible silver source (see, for example, U.S. Pat. No. 3,839,049). The silver halide may also be made "ex situ" and added to the organic silver salt. The addition of silver halide grains to photothermographic elements is described in Research Disclosure, June 1978, Item No. 17029. It is also reported in the art that when silver halide is made ex situ, one has the possibility of controlling the composition and size of the grains much more precisely, so that one can impart more specific properties to the photothermographic element and can do so much more consistently than with the in situ technique.
The non-photosensitive, reducible silver source is a compound that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic compounds, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further processed to produce a visible image. This is accomplished by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, (i.e., the latent image). This produces a black and white image. In photographic elements, the silver halide is reduced to form the black-and-white image. In photothermographic elements, the light-insensitive silver source is reduced to form the visible black-and-white image while much of the silver halide remains as silver halide and is not reduced.
In photothermographic elements the reducing agent for the organic silver salt, often referred to as a "developer," may be any compound, preferably any organic compound, that can reduce silver ion to metallic silver. At elevated temperatures, in the presence of the latent image, the non-photosensitive reducible silver source (e.g., silver behenate) is reduced by the reducing agent for silver ion. This produces a negative black-and-white image of elemental silver.
While conventional photographic developers such as methyl gallate, hydroquinone, substituted-hydroquinones, catechol, pyrogallol, ascorbic acid, and ascorbic acid derivatives are useful, they tend to result in very reactive photothermographic formulations and fog during preparation and coating of photothermographic elements. As a result, hindered phenol developers (i.e., reducing agents) have traditionally been preferred.
As the visible image in black-and-white photothermographic elements is usually produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable to reduce the cost of raw materials used in the emulsion and/or to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic elements as described in U.S. Pat. Nos. 3,846,136; 3,994,732; and 4,021,249.
Another method of increasing the maximum image density in photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming or dye-releasing compounds in the emulsion. Upon imaging, the dye-forming or dye-releasing compound is oxidized, and a dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye-enhanced black-and-white silver image can be produced. Dye enhanced black-and-white silver image forming compounds and processes are described in U.S. Pat. No. 5,185,231.
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic elements differ significantly from conventional silver halide photographic elements which require wet-processing.
In photothermographic imaging elements, a visible image is created by heat as a result of the reaction of a developer incorporated within the element. Heat is essential for development and temperatures of over 100.degree. C. are routinely required. In contrast, conventional wet-processed photographic imaging elements require processing in aqueous processing baths to provide a visible image (e.g., developing and fixing baths) and development is usually performed at a more moderate temperature (e.g., 30.degree.-50.degree. C.).
In photothermographic elements only a small amount of silver halide is used to capture light and a different form of silver (e.g., silver behenate) is used to generate the image with heat. Thus, the silver halide serves as a catalyst for the development of the non-photosensitive, reducible silver source. In contrast, conventional wet-processed black-and-white photographic elements use only one form of silver (e.g., silver halide) which, upon development, is itself converted to the silver image. Additionally, photothermographic elements require an amount of silver halide per unit area that is as little as one-hundredth of that used in conventional wet-processed silver halide.
Photothermographic systems employ a light-insensitive silver salt, such as silver behenate, which participates with the developer in developing the latent image. In contrast, photographic systems do not employ a light-insensitive silver salt directly in the image-forming process. As a result, the image in photothermographic elements is produced primarily by reduction of the light-insensitive silver source (silver behenate) while the image in photographic black-and-white elements is produced primarily by the silver halide.
In photothermographic elements, all of the "chemistry" of the system is incorporated within the element itself. For example, photothermographic elements incorporate a developer (i.e., a reducing agent for the non-photosensitive reducible source of silver) within the element while conventional photographic elements do not. The incorporation of the developer into photothermographic elements can lead to increased formation of "fog" upon coating of photothermographic emulsions. Even in so-called instant photography, the developer chemistry is physically separated from the silver halide until development is desired. Much effort has gone into the preparation and manufacture of photothermographic elements to minimize formation of fog upon coating, storage, and post-processing aging.
Similarly, in photothermographic elements, the unexposed silver halide inherently remains after development and the element must be stabilized against further development. In contrast, the silver halide is removed from photographic elements after development to prevent further imaging (i.e., the fixing step).
In photothermographic elements the binder is capable of wide variation and a number of binders are useful in preparing these elements. In contrast, photographic elements are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic elements require thermal processing, they pose different considerations and present distinctly different problems in manufacture and use. In addition, the effects of additives (e.g., stabilizers, antifoggants, speed enhancers, sensitizers, supersensitizers, etc.) which are intended to have a direct effect upon the imaging process can vary depending upon whether they have been incorporated in a photothermographic element or incorporated in a photographic element.
Because of these and other differences, additives which have one effect in conventional silver halide photography may behave quite differently in photothermographic elements where the underlying chemistry is so much more complex. For example, it is not uncommon for an antifoggant for a silver halide systems to produce various types of fog when incorporated into photothermographic elements.
Distinctions between photothermographic and photographic elements are described in Imaging Processes and Materials (Neblette's Eighth Edition); J. Sturge et al. Ed; Van Nostrand Reinhold: New York, 1989; Chapter 9 and in Unconventional Imaging Processes; E. Brinckman et al, Ed; The Focal Press: London and New York: 1978; pp. 74-75.
Various techniques are typically employed to try and gain higher sensitivity in a photothermographic element. In efforts to make more sensitive photothermographic elements, one of the most difficult parameters to maintain at a very low level is the various types of fog or Dmin. Fog is spurious image density which appears in non-imaged areas of the element after development and is often reported in sensitometric results as Dmin. Photothermographic emulsions, in a manner similar to photographic emulsions and other light-sensitive systems, tend to suffer from fog.
Traditionally, photothermographic elements have suffered from fog upon coating. The fog level of freshly prepared photothermographic elements will be referred to herein as initial fog or initial Dmin.
In addition, the fog level of photothermographic elements often rises as the element is stored, or "ages." This type of fog will be referred to herein as shelf-aging fog. Adding to the difficulty of fog control on shelf-aging is the fact that the developer is incorporated in the photothermographic element. This is not the case in most silver halide photographic systems. A great amount of work has been done to improve the shelf-life characteristics of photothermographic elements.
A third type of fog in photothermographic systems results from the instability of the image and/or background after processing. The photoactive silver halide still present in the developed image may continue to catalyze formation of metallic silver during room light handling or post-processing exposure such as in graphic arts contact frames. This is known as "post-processing fog" or "silver printout." Without having acceptable resistance to fog, a commercially useful element is difficult to prepare. Thus, there exists a need for "print stabilizers" to stabilize the unreacted silver halide. Various techniques have been employed to improve sensitivity and maintain resistance to fog.
In color photothermographic elements, often unreacted dye forming or dye releasing compounds may slowly oxidize and form areas of color in the unexposed areas. In these elements, stabilizers are often added to reduce "leuco dye backgrounding."
The addition of separate post-processing image stabilizers or stabilizer precursors provides the desired post-processing stability. Most often these are sulfur-containing compounds such as mercaptans, thiones, and thioethers as described in Research Disclosure, June 1978, item 17029. U.S. Pat. No. 4,245,033 describes sulfur compounds of the mercapto-type that are development restrainers of a photothermographic system. See also U.S. Pat. Nos. 4,837,141 and 4,451,561. Mesoionic 1,2,4-triazolium-3-thiolates as fixing agents and silver halide stabilizers are described in U.S. Pat. No. 4,378,424. Substituted 5-mercapto-1,2,4-triazoles, such as 3-amino-5-benzothio-1,2,4-triazole, used as post-processing stabilizers are described in U.S. Pat. Nos. 4,128,557; 4,137,079; 4,138,265; and Research Disclosure 16977 and 16979.
Some of the problems with these stabilizers include thermal fogging during processing or losses in photographic sensitivity, maximum density, or contrast at effective stabilizer concentrations.
Stabilizer precursors have blocking or modifying groups that are cleaved during processing with heat, light, and/or alkali. This provides an active stabilizer that can combine with the photoactive silver halide in the unexposed areas of the photographic element to form a light- and heat-stable complex. For example, in the presence of a stabilizer precursor in which the blocking group on a sulfur atom is cleaved upon processing, the resulting silver mercaptide will be more stable than silver halide to light, and heat.
Blocking groups that are thermally-sensitive have also been used. These blocking groups are removed by heating the imaging element during processing. For example, U.S. Pat. No. 5,158,866 describes the use of omega-substituted 2-propionamidoacetal or 3-propionamidopropionyl stabilizer precursors as post-processing stabilizers in photothermographic elements. U.S. Pat. No. 5,175,081 describes the use of certain azlactones as stabilizers. U.S. Pat. No. 5,298,390 describes the use of certain alkyl sulfones as blocked compounds capable of releasing stabilizers with heat. U.S. Pat. No. 5,300,420 describes the use of certain nitriles as blocked compounds capable of releasing stabilizers with heat.
Various disadvantages attend these different blocking techniques. Highly basic solutions that are necessary to cause unblocking of the alkali-sensitive blocked derivatives are corrosive and irritating to the skin. With photographic stabilizers that are blocked with a heat-removable group, it is often found that the liberated reagent or by-product can react with other components of the photothermographic element and cause adverse effects. Also, premature release of the stabilizing moiety within the desired time during processing may occur, resulting in fogging of the emulsion or loss of sensitivity.
It is known that, DNA, RNA, and their decomposition products are photographic restrainers which differ from one another in efficiency. As described by Steigmann (Sci. Indust. Photogr. 1964, 35, 145) and Ammann-Brass (J. Photogr. Sci. 1972, 20, 37) the physical and chemical ripening of silver halide is more strongly inhibited by partially decomposed DNA than by the undestroyed double-strand DNA or the mononucleotides and mononucleosides. Also the products of full decomposition of the DNA have a slow restraining action smaller than that of the intact DNA. Likewise, the oligonucleotides have little or even no effect on ripening. It is also known (G. B. Tagliafico, J. Photogr. Sci. 1969, 17, 17 that when DNA is completely decomposed into oligonucleotides, has no restraining effect on the chemical ripening of an ammoniacal silver bromide emulsion either. These experiments indicate that evidently the optimum restraining of DNA can be obtained with its partially decomposed molecules. A recent study (H. Hermel and A. Huttner J. Imag. Sci. Tec. 1992, 36 (3),287) shows that partially decomposed DNA acts as antifoggant and sensitizer in the chemical ripening process. Similar results have been observed when partially decomposed DNA prepared by ultrasonic treatment was added to an X-ray emulsion, a negative emulsion, a direct positive emulsion, and a reprolith emulsion during the chemical ripening process.
There is a continued need for improved stabilizer compounds that inhibit all types of fog and do not have any detrimental effects on the photothermographic element.